This invention relates to a driographic ink which is suitable for use in driography, or waterless planographic printing.
Lithography is the best known form of planographic printing, i.e., wherein both image and background areas lie substantially in the same plane. As such, lithography has been the only known practical and successful process of planographic printing heretofore known. Lithography works on the basic theory that water and oil are immisible, and the background or non-image areas of a lithographic printing plate are rendered water-receptive, i.e., hydrophilic, and when water-wet they thus repel conventional oily lithographic ink. The image areas, conversely, are ink-receptive, i.e., organophilic, and water-repellent. In operation, the plate is typically first dampened with a fountain solution, which wets the background or non-image areas, after which the oleo ink is rolled over the plate to coat the image areas, but is repelled from the dampened background areas.
While lithography has enjoyed tremendous commercial success, it has not been without attendant problems. One of the major problems with lithography is the basic fountain solution/ink combination. The necessity of the fountain solution can cause emulsification of the oily ink, and the fountain solution, because of its contact with the offset cylinder, can cause moistening of the copy sheets, thereby changing the dimensions thereof. Such can create special difficulties in the area of color printing, wherein each copy sheet must of necessity undergo multiple passes through the press.
The biggest problem is the control of the delicate balance necessary between ink and fountain solution so as to produce high quality image fidelity and uniformity. Such a delicate balance is difficult to maintain, and must be constantly monitored, especially as conditions change on the press during the course of the printing run.
In order to overcome the inherent problems relating to lithography, several dry planographic printing masters have appeared in the patent literature, which are intended to circumvent the aforementioned problems. A driographic printing plate, which removes the necessity for utilization of a dampening system, contains background or non-image areas which are inherently ink-repellent. Therefore, in theory, only the imaged portions of the driographic plate will accept ink. One such printing plate is disclosed and claimed in U.S. Pat. No. 3,511,178.
In essence, driography is based on the adhesion properties of the driographic plate. The plate typically has a background surface having a sufficiently low adhesion to printing ink that, without pre-wetting the plate with a dampening solution, the ink that is applied thereto in such areas will not split away and transfer from the inking rollers to the plate. In other words, the adhesion of the ink to the rollers and the cohesive forces between the ink particles are both greater than the adhesion between the ink and the plate surface.
In practice, however, driographic masters have been found to exhibit a different set of problems on the printing press, such problems having been found to be essentially as difficult to control as the aforementioned problems with lithography. Basically, the problems relating to driography are ink related. Because the background composition of the plates is inherently ink-repellent, the inks utilized with such plates must be carefully compounded so as to not deposit in the background areas, while at the same time, the same ink must readily deposit from the rollers to the printing or image areas of the master.
When conventional lithographic type inks are compounded for utilization with driographic masters, it has been ascertained that the cohesive properties of the ink must be increased to prevent ink deposition in the background areas. However, the cohesive nature of the ink must be carefully balanced so as to allow for deposition of ink on the image areas of the master as well. This careful balance of ink cohesive and adhesive properties can be easily disrupted by ambient temperature conditions, heating of the ink due to viscous flow and ink film splitting in the ink train, and by the breaking down of the ink filler structure due to mechanical working. The increase of ink temperature via viscous flow and ink film splitting is particularly troublesome because of the greater cohesive nature of the ink, which allows for greater energy dissipation in the ink train.
At reduced ink temperatures, the driographic master will not be properly coated or inked in the image areas. Because the ink is of a tacky nature, it may also pick fibers from the copy sheet during the offset process. Such may result in incomplete transfer of information to the copy sheet. Conversely, at elevated temperatures the ink will deposit in both the image and background areas of the master. The resultant information on the copy sheet is thereby rendered difficult to read because of the overall deposition of ink.
It has been generally found that when using conventional lithographic inks with driographic masters, ink film temperatures must be controlled within a temperature range of from about 10.degree. F. to about 30.degree. F. in order to satisfy printing requirements. This range can, of course, be easily exceeded by variations in ambient temperatures in the press room. For example, ink film temperatures may often vary by over 50.degree. F. from a cold start of the press in the morning at 55.degree. F. to approximately 110.degree. F. after several hours of press operation.
Conventional printing inks are typically mixtures of varnishes, pigments, oils, solvents, driers, and other minor components. Inks utilized in offset printing are typically compounded to be highly viscous, i.e., from about 50 to about 500 poise at 90.degree. F., have a high yield value and are termed "short". By the term "short" is meant that when an ink film is split, the film will break without forming long threads or strands. Such long threads or strands are considered undesirable because same have been found to cause misting or cobwebbing on the copy sheet. By cobwebbing is meant that poor edge sharpness of a print is observed on the copy sheet, which is a result of the fine fibers of ink extending from the ink images on the copy sheet. Therefore, viscosity and shortness have been thought to be important rheological properties for the preparation and compounding of inks, as is taught by J. H. Taylor and A. C. Zettlemoyer, TAPPI, Volume 41, No. 12 (1958) at page 749.
These rheological properties are typically controlled to a considerable degree by the varnish component contained in the ink. Varnishes are solutions or dispersions of polymers in hydrocarbon or other solvent, or in a drying oil such as linseed oil. The polymer/solvent combinations are typically chosen so that the ink is short and also contains a high weight percent of polymer. Low molecular weight polymers are utilized since it is desired to keep the varnish viscosities low and the polymer concentrations high in order to obtain proper ink splitting properties in the ink train, as well as a durable gloss image on paper. Such inks, when utilized in driography, have been unable to solve the aforementioned problems relative to the heat dissipation within the ink, causing same to increase in temperature beyond their acceptable printing ranges.
One driographic ink, disclosed in Belgian Publication 842,646, is designed to provide for a high viscosity and also to not become pseudoplastic in its behavior at elevated press temperatures, generally from about 85.degree. F. to about 125.degree. F. and preferably even higher. The elevated viscosity of this ink is taught to be achieved by incorporating therein high concentrations of conventional ink resins to thereby reduce the response of the ink viscosity to increased temperature. Although such an approach is helpful in stabilization of ink viscosity, it can also be self-defeating because the high ink viscosity at elevated temperatures generates greater heat during press operation.
Surprisingly it has now been found that by compounding a driographic ink utilizing the elasticity thereof as opposed to the viscosity as the controlling rheological property, together with the utilization of hereinafter defined fillers, there can be provided an ink having temendous temperature latitude. Additionally, by incorporation therein of a weak boundary layer fluid, the temperature range within which a given ink can function can be shifted.